NOx contained in exhaust gases or flue gases discharged from power plants, various factories, automobiles, and others are causative agents for photochemical smog and acid rain. As an efficient method for removing the NOx, an exhaust gas or flue gas denitration method by a selective catalytic reduction using ammonia (NH3) as reducing agent has widely been employed with thermal power plants being the places where the method is most frequently used. As the catalyst used for such an exhaust gas or flue gas denitration method, a titanium oxide (TiO2) type catalyst containing vanadium (V), molybdenum (Mo), or tungsten (W) as an active component has been used. Especially, a catalyst containing vanadium as one active component has become a mainstream of current denitration catalysts since the catalyst is not only high in activity but also small in deterioration due to the impurities contained in the exhaust gases and usable at temperatures including temperatures lower than those used heretofore (Laid-open Japanese Patent Publication No. Sho 50-128681 and others). The catalyst has been used after being molded usually into a honeycomb-like or platelike shape, and various methods for producing the catalyst has been devised.
Besides, a fact that dioxins having a high toxicity are contained in exhaust gases discharged from incinerators burning municipal refuses or industrial wastes has become a social problem in recent years. Thus, the catalysts which perform a denitration reaction and oxidatively decompose dioxins at the same time have been invented.
Many of the catalysts described above are ordinarily prepared by (i) a method in which particles of a titanium oxide, and particles of salts of active components such as V, Mo, and W of a catalyst or particles of an oxide are kneaded together with water, and the mixture thus obtained is molded and calcined (kneading method), or (ii) another method in which a molded and calcined article of a titanium oxide is impregnated with a solution of a mixture of the salts of active components of a catalyst (impregnation method) (Laid-open Japanese Patent Publication No. Sho 50-128681, Japanese Patent Publication No. Sho 53-34195, and others).
The kneading method and impregnation method described above both of which fall within conventional technology can not always be said to be best methods for preparing the catalysts when viewed from the aspect of the activity of catalysts, and many angles undesirable exist in the methods especially when the catalyst having a high activity at low temperatures are to be obtained.
The problems contained in the methods are enumerated with the problems being separated into those belong to kneading method or those belong to the impregnation method as follows:
1) Kneading Method
                {circle around (1)} In order to obtain a catalyst (final product) having a high activity, it is necessary to activate added salts of active components contained in a molded catalyst through calcination thereof. However, since a titanium oxide and active components contained in the molded catalyst are sintered by the calcination, it is difficult to obtain a catalyst (final product) having a high activity at low temperatures.        {circle around (2)} So-called compositing effects of V with Mo or W is not sufficient because particles of a titanium oxide, and salts of active components such as V, Mo, and W, or particles of an oxide once become a state in which all of them coexist as they are by the kneading, and then they are composited only after they were subjected to a calcination. Accordingly, the kneading method leads to the formation of a catalyst (final product) having a small improvement in the durability and having a small resistance to SOx.2) Impregnation Method        {circle around (1)} Like the kneading method, it is necessary to activate the active components by calcining a molded catalyst. Accordingly, it is impossible to avoid the active components from being sintered, and thus a catalyst having a high activity, especially a high activity at low temperatures can not be obtained.        {circle around (2)} Since a catalyst molded and calcined in advance is impregnated with active components in the impregnation method, compositing of the active components become easy compared with the kneading method. On the other hand, however, the concentrations of active components become different between the inside and the surface of a catalyst, it is difficult to maintain the ratio of two or more active components within a catalyst at a constant value, and thus most suitable compositing effects can not be expected, because the active components are adsorbed by a titanium oxide in the process of the impregnation.        
In order to overcome the problems in the background art described above,                (i) combined methods of a kneading method with an impregnation method such as an impregnation method in which a titanium oxide-molybdenum oxide carrier is impregnated with a vanadium salt and another impregnation method in which a titanium oxide carrier is impregnated sequentially with W, V, and others; and        (ii) improvements in the impregnation method have been attempted.        
However, it can hardly be said that sufficient-effects can be obtained by those methods.